1. Field of the Invention
The present invention relates to an electric power steering device.
2. Discussion of the Related Art
FIG. 8 illustrates a conventional electric power steering device as described in Japanese laid-open patent publication number 2000-280920 and Japanese laid-open patent publication number 2000-280923.
In the prior art electric power steering device, housings 91, 92 are attached by a bolt 93, and a rack bar 94 is disposed in the housings 91, 92. The housing 91 is formed with a motor attachment section 91a that projects diagonally relative to the rack bar 94. A motor 95 is secured to the motor attachment section 91a, and the motor shaft 95a of the motor 95 is in a diagonally intersecting relationship with the rack bar 94. Power transmitting means is disposed between the motor shaft 95a and the rack bar 94. More specifically, a drive gear 96, in the form of a bevel gear with teeth extending in a linear direction, forms a serration fit or a spline fit with the motor shaft 95a. A shaft section 96a of the drive gear 96 is rotatably supported by bearings 97 disposed between it and the motor attachment section 91a. A spacer 98 is interposed between a head 96b of the drive gear 96 and the inner ring of the bearing 97. Also, a ball screw mechanism 99 is disposed coaxially on the rack bar 94, and a nut 99a of the ball screw mechanism 99 is rotatably supported by a bearing 82 disposed between it and the housing 92. A driven gear 82 is secured to the nut 99a of the ball screw mechanism 99 and a driven gear 82 meshes with the drive gear 96. Driven gear 82 is formed as a bevel gear having teeth extending in a linear direction. A spacer 83 is interposed between the driven gear 82 and the inner ring of the bearing 81.
In the prior art electric power steering device of FIG. 8, the motor shaft 95a of the motor 95 is rotated based on steering torque generated by a steering shaft not shown in the figures. The rotational drive force of the motor shaft 95a is transferred to the nut 99a of the ball screw mechanism 99 by the drive gear 96 and the driven gear 82. The rotational motion of the nut 99a is converted by the ball screw mechanism 99 into longitudinal linear motion of the rack bar 94. Thus, the drive gear 96, the ball screw mechanism 99, and the driven gear 82, which serve as power transmission means transfer the rotational drive force from the motor shaft 95a to the rack bar 94 as auxiliary propulsion. As a result, the rotational motion of the steering shaft is converted to longitudinal linear motion of the rack bar 94 with the assistance of the motor 95, thus allowing the steering angle to be changed. Also, the spacer 98 disposed at the motor 95 and the spacer 83 disposed at the ball screw mechanism 99 serve to adjust the backlash between the drive gear 96 and the driven gear 82.
This type of conventional electric power steering device has the spacer disposed at the motor and the spacer disposed at the ball screw which provides stepwise adjustments in backlash. Non-stepped adjustments in backlash are not possible between the drive gear and the driven gear. More specifically, the backlash between the drive gear and the driven gear cannot be adjusted unless multiple spacers having different thicknesses are prepared and mounted in different combinations until the drive gear and the driven gear are at predetermined dimensions from a reference position determined ahead of time. Thus, the prior art electric power steering device involves tedious assembly work as well as a larger number of maintenance parts and higher production costs.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.